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G8MNY > TECH 07.10.12 21:19l 111 Lines 4908 Bytes #999 (0) @ WW
BID : 61771_GB7CIP
Read: GUEST
Subj: Band 2 6el Broadband Yagi
Path: IZ3LSV<IW8PGT<ON0AR<VK6HGR<GB7CIP
Sent: 121007/1908Z @:GB7CIP.#32.GBR.EU #:61771 [Caterham Surrey GBR] $:61771_GB
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Sep 12)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
On looking at an large old well made home-made 6el broadcast Yagi beam (scrap)
as a starting point, I decided to re-model it using the MMANA-GAL program
(CD in 2010 RSGB yearbook & online). It showed it to be rubbish as it was!
The community radio station I put on, is too far to hear at my home really, so
I needed a good aerial. I wanted it to do well over the whole 20MHz of Band 2,
87.5-108MHz, with good front to back ratio & have some useful gain.
Using the optimisation over frequencies near the edge of the band, I ended up
with this quite acceptable compromise in 20mm ally tube for the elements, &
boom was 40mm & 2.54m long!
2x
849 4x
³cm 732
³ Ú¿cm 2x
³ ³³ 647 2x
³ ³³ ³cm 614 2x
³ ³³ ³ ³cm 583 2x
³ ³³ ³ ³ ³cm 522
³ ³³ ³ ³ ³ ³cm
³ ³³ ³ ³ ³ ³
³ ³³ ³ ³ ³ ³
³ ³³ ³ ³ ³ ³
ÃÄÄÄÄÄÄÄÅ0ÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ´
³ 326 ³³186³ 713 ³ 550 ³ 758 ³
³ cm ³³cm ³ cm ³ cm ³ cm ³
³ ³³ ³ ³ ³ ³
³ ³³ ³ ³ ³ ³
³ ³³ ³ ³ ³
³ ³³ ³ ³
³ ³³ ³
³ ³³
³ ÀÙ
³ 98mm dia
folded ends.
Freq Gain F/B R J
88 7.9 14.5 97 -4 _-~~~-_
90 7.7 12.8 146 41 / \
92 7.6 11.7 198 71 Typical Þ Ý
94 7.7 11.1 252 86 polar \ /
96 7.8 11.0 308 82 diagram \_ _/
98 8.0 11.0 355 52 across >ú<
100 8.3 11.3 375 -2 whole ( )
102 8.6 11.7 346 -64 band. -.-
104 9.0 12.4 263 -89
106 9.6 13.6 165 -49
108 10.1 15.6 87 47
AERIAL Z
The impedance of this folded dipole feed point is approx. 300 Ohms balanced
over most of the band. This is not normal as a folded dipole increases the
impedance by 4 & the reflector & director usually reduces it by 4, resulting
the usual 75ê. But this design is optimised for bandwidth, so we to do extra
matching, to get 75ohm. I used a small centre tapped VHF 2 hole ferrite bread
as the balun, with 2x 2 turns on it all in the connection box.
IN GRAPHICS
SWR 300ê .==========================.
³ ((Folded Dipole ))
3´\ ³ '========(0) (0)========='
³ \ / Ã())))Ù
2´ \ / TRANSFORMER ³ ³___
³ '-.______ _.-' 4:1 BALUN ÀÄo)coax
1Å ~~~~~~~~~ ~~~~~
87.5 92.75 97.5 102.5 107.5MHz
R ê X ê
400´ __ 200´
³ .-~' ~-. ³
300´ .-~ \ 100´ _...._
³ .-~ \ ³ .ú'~ 'ú.
200´ .-~ '. 0´.' \ /
³.-~ . ³ 'ú._ _.ú'
100´ . -100´ ~
87.5 92.75 97.5 102.5 107.5MHz 87.5 92.75 97.5 102.5 107.5MHz
GAIN dBd F/B dB
10´ / 15´_ _
³ / ³ -. _.-'
9´ .' 12.5´ 'ú. _.-'
³ _.-' ³ '-..__..-'~
8´~-. _.ú'~ 10´
³ '---....-ú' ³
7´ 7.5´
87.5 92.75 97.5 102.5 107.5MHz 87.5 92.75 97.5 102.5 107.5MHz
Z MEASUREMENTS
Using an MFJ analyser, the match to 75ê is pretty good over most of the band as
predicted by the model. I hope when I put it up on a rotator it works as
predicted too.
IN USE
This works well 75ê SWR was quite flat as above. But now regret I did not
designed it to be a spot frequency narrow band 1MHz wide aerial, as that would
give 2-3dB more gain & > 22dB F/B ratio. The back of the beam QRM is the main
problem I have!
See My Tech Bul "Band 2 6el Narrow Band Yagi" version!
Why don't U send out an interesting bul?
73 De John, G8MNY @ GB7CIP
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